This application aims to investigate the role of the UL97 protein kinase of human cytomegalovirus (HCMV) in viral replication, with an explicit focus on nuclear egress. UL97 has been implicated in the phosphorylation of lamin A/C, a cellular protein considered relevant to herpesvirus replication at the stage of nuclear egress. UL97 was originally identified as the source of ganciclovir kinase activity in HCMV-infected cells, but it appears that LJL97 is more likely to function as a protein serine-threonine kinase than as a nucleoside kinase. Maribavir, a specific inhibitor of UL97 kinase activity, has shown considerable promise in clinical trials, affirming the medical importance of UL97 as an antiviral drug target for treatment of HCMV infection. The growth defect of UL97 deletion virus (A97) has been previously attributed to impaired DNA replication and capsid assembly, or to a block at the stage of nuclear egress. Interestingly, lamin A/C, a major component of the nuclear lamina, specifically undergoes UL97-dependent changes in its phosphorylation pattern during HCMV infection. Since A97 virus has defects in nuclear egress, and moreover fails to induce characteristic deformations of the nuclear lamina commonly seen in cells infected with wild-type HCMV, this project will test the hypothesis that UL97 coordinates the disassembly of lamin A/C polymers to promote nuclear egress. The project will examine whether UL97 can disassemble polymers of A-type nuclear lamins in vitro, and whether cells lacking A-type lamins can complement the growth defect of A97 viruses. The project will also test if UL97 can directly cause nuclear lamina alterations during HCMV infection, or whether cellular kinases are also required. UL97 has recently been identified as the kinase responsible for phosphorylation of the retinoblastoma tumor suppressor (Rb) in HCMV-infected cells. In normal cells, Rb phosphorylation can cause upregulation of several host cell genes that may benefit HCMV replication, including genes involved in DNA replication and phosphorylation of lamin A/C. Therefore, this application will also test whether an Rb-degrading protein from human papillomavirus can substitute for UL97 to promote HCMV replication in cultured cells. As UL97 is related to kinases conserved among several medically important herpesviruses, this application aims to add to our basic understanding of herpesvirus biology. Furthermore, since HCMV is a leading cause of both congenital defects in newborn children, and of life-threatening disease in immunocompromised members of the U.S. population, this research also offers to contribute to a better understanding of an important therapeutic drug target.